What sex?

Half of your genes come from your father and half come from your mother. They are bundled up as a set of 46 chromosomes. One pair of these, the sex chromosomes, determines what sex you are. An embryo with one X- and one Y-chromosome will be a boy, and one with two X-chromosomes will be a girl. The presence of a Y-chromosome turns on a 'male switch' in the developing embryo.

What makes a man?

In a seven-week-old embryo, the sex glands and organs of males and females appear identical. In male embryos, the testis determining factor gene, called SRY, is then switched on. This 'male gene', found on the Y-chromosome, triggers male development in all mammals and causes the testes and male external genitals to start to grow. In a female embryo, there is no Y-chromosome and no SRY gene, allowing the ovaries and female external genitals to develop. Other genes are also involved in sex determination.